Steel and mould tool for plastic materials made of the steel

ABSTRACT

The invention concerns a steel, particularly a steel for mould tools for plastic moulding, having the following chemical composition in weight-%:0.43-0.60 C from traces to 1.5 Si from traces to 1.5 (Si+Al) 0.1-2.0 Mn 3.0-7.0 Cr 1.5-4.0 (Mo+), however max. 1.0 W 0.30-0.70 V max. 0.1 of each of Nb, Ti and Zr max. 2.0 Co max. 2.0 Ni balance essentially only iron and unavoidable impurities. After hardening and high temperature tempering at 520-560° C., the steel has a hardness of 56-68 HRC.

TECHNICAL FIELD

The invention relates to a steel, i.e. an alloy, intended to be used inthe first place for the manufacturing of mould tools in which plasticproducts shall be manufactured by some kind of moulding method in theplastic or moulded condition of the plastic material. The invention alsorelates to tools and tool details made of the steel, and blanks of thesteel alloy for the manufacturing of mould tools for plastic materialsand details for such tools.

BACKGROUND OF THE INVENTION

Mould tools for plastic materials are made of a great number of varioussteel alloys, including martensitic, medium alloyed steels. In thatgroup there is a commercially available steel which nominally contains0.6% C, 4.5% Cr, 0.5% Mo and 0.2% V and which is used for cold worktools and mould tools for plastic materials. Within the same group thereis also found the standardised steel AISI S7 which is also sometimesused for inter alia mould tools for moulding plastic materials, andanother commercial available tool steel, which nominally contains 0.55%C, 2.6% Cr, 2.25% Mo and 0.9% V. The two first named steels attained adesired hardness only after low temperature tempering, which may causerisk for retained tensions in the steel after heat treatment. It is truethat the last mentioned steel may achieve an adequate hardness afterhigh temperature tempering, i.e. tempering at about 550° C., on theother hand the hardenability of that steel is not particularly good.

It is the purpose of the invention to provide a mould steel for mouldingplastic materials which has a better combination of features for theemployment of the steel for the manufacturing of mould tools for plasticmaterials, than the tool steels which presently are commerciallyavailable. Particularly, the steel should have the following features:

-   -   Good ductility/toughness,    -   Good hardenability allowing through hardening in connection with        conventional hardening in a vacuum furnace of products with        thicknesses up to at least 350 mm,    -   Adequate hardness, at least 54 HRC, preferably at least 56 HRC,        after hardening and high temperature tempering, which gives a        high resistance against plastic deformation and, at least as far        as certain applications are concerned, also an adequate wear        resistance without nitriding or surface coating with titanium        carbide and/or titanium nitride or the like by means of e.g.        PVD- or CVD-technique,    -   Good tempering resistance in order to allow nitriding or surface        coating with titanium carbide and/or titanium nitride or the        like by e.g. any of said techniques without reduction of the        hardness of the material for applications which require        particularly good wear resistance of the tool,    -   Good heat treatment features,    -   Good grindability, machinability by cutting operations, spark        machinability, and polishability.

Other important product features are:

-   -   Good dimension stability during heat treatment,    -   Long fatigue life.

Specifically, the invention aims at providing a matrix steel which canbe employed as a material for mould tools for plastic materials, i.e. asteel which is essentially void of primary carbides and which in its usecondition has a matrix consisting of tempered martensite.

DISCLOSURE OF THE INVENTION

The above mentioned purposes and features can be achieved by means of asteel which is characterised by what is stated in the appending patentclaims.

As far as the individual elements of the steel alloy and their mutualinteraction are concerned, the following applies. Percentages mentionedin this text always refer to weight-% if not otherwise is stated.

The steel of the invention shall, as above mentioned, not contain anyprimary carbides but nevertheless have a wear resistance which isadequate for most applications. This is achieved by an adequate hardnesswithin the range 54-59 HRC, suitably 56-58 HRC, in the hardened and hightemperature tempered condition of the steel, at the same time as thesteel shall have a very good toughness. In order to achieve this, thesteel contains carbon and vanadium in well balanced amounts. Thus thesteel should contain at least 0.43%, preferably at least 0.44%, andsuitably at least 0.46% C. Further the steel should contain at least0.30%, preferably at least 0.40%, and suitably at least 0.45% V in orderto ensure that the martensitic matrix of the steel in the hardened andtempered condition of the steel, shall contain a sufficient amount ofcarbon in solid solution in order to give the matrix said hardness andalso in order that an adequate amount of secondarily precipitated, verysmall hardness increasing vanadium carbides shall be formed in thematrix of the steel. Moreover, very small, primary precipitated vanadiumcarbides exist in the steel, which contribute to the prevention of graingrowth during the heat treatment. Any other carbides than vanadiumcarbides should not exist. In order to achieve said conditions, thesteel must not contain more than 0.60%, preferably max. 0.55%, andsuitably max. 0.53% C, and max. 0.70%, preferably max. 0.65%, andsuitably max. 0.60% V. Nominally, the steel contains 0.49% C and 0.52%V. The amount of carbon in solid solution in the hardened and hightemperature tempered condition of the steel nominally amounts to about0.45%.

Silicon exists at least in a measurable amount as a residual elementfrom the manufacturing of the steel and is present in an amount fromtraces up to max. 1.5%. Silicon, however, impairs the toughness of thesteel and should therefore not exist in an amount above 1.0%, preferablymax. 0.5%. Normally, silicon exists in a minimum amount of at least0.05%. An effect of silicon is that it increases the carbon activity inthe steel and therefore contributes to affording the steel a desiredhardness. Therefore it may be advantageous that the steel containssilicon in an amount of at least 0.1%. Nominally the steel contains 0.2%silicon.

Aluminium to some extent may have the same or similar effect as siliconat least in a steel of the present type. Both can be used as oxidationagents in connection with the manufacturing of the steel. Both areferrite formers and may provide a dissolution hardening effect in thematrix of the steel. Silicon therefore may be partly replaced byaluminium up to an amount of max. 1.0%. Aluminium in the steel, however,makes it necessary that the steel is very well deoxidised and has a verylow content of nitrogen, because aluminium oxides and aluminium nitridesotherwise would form, which would reduce the ductility/toughness of thesteel considerably. Therefore, the steel should normally not containmore than max. 1.0% Al, preferably max. 0.3%. In a preferred embodiment,the steel contains max. 0.1% and most conveniently max. 0.03% Al.

Manganese, chromium and molybdenum shall exist in a steel in asufficient amount in order to give the steel an adequate hardenability.Manganese also has the function of binding the extremely low contents ofsulphur which may exist in the steel to form manganese sulphides.Manganese therefore, shall exist in an amount of 0.1-2.0%, preferably inan amount of 0.2-1.5%. Suitably, the steel contains at least 0.25% andmax. 1.0% manganese. A nominal manganese content is 0.50%.

Chromium shall exist in a minimum amount of 3.0%, preferably at least4.0% and suitably at least 4.5% in order to give the steel a desiredhardenability when the steel contains manganese and chromium in amountswhich are characteristic for the steel. Maximally, the steel may contain7.0%, preferably max. 6.0% and suitably max. 5.5% chromium.

Also molybdenum shall exist in an adequate amount in the steel in orderto afford, together with in the first place chromium, the steel adesired hardenability and also to give it a desired secondary hardening.Molybdenum in too high contents, however, causes precipitation of M6Ccarbides, which preferably should not exist in the steel. With thisbackground, the steel therefore shall contain at least 1.5% and max.4.0% Mo. Preferably, the steel contains at least 1.8% and max. 3.2% Mo,suitably at least 2.1% and max. 2.6% Mo in order that the steel shallnot be caused to contain undesired M6C carbides at the cost of and/or inaddition to the desired amount of MC carbides. Molybdenum in principalcompletely or partly may be replaced by tungsten for the achievement ofa desired hardenability, but this requires twice as much tungsten asmolybdenum which is a drawback. Also recirculation of scrap which isproduced in connection with the manufacturing of the steel is made moredifficult if the steel contains substantial contents of tungsten.Therefore, tungsten should not exist in an amount of more than max.1.0%, preferably max. 0.3%, suitably max. 0.1%. Most conveniently, thesteel should not contain any intentionally added amount of tungsten,which in the most preferred embodiment of the steel should not betolerated more than as an impurity in the form of a residual elementemanating from used raw materials for the manufacturing of the steel.

In addition to the said elements, the steel normally need not containany further, intentionally added alloy elements. Cobalt, for example, isan element which normally is not required for the achievement of thedesired features of the steel. However, cobalt may optionally be presentin an amount of max. 2.0%, preferably max. 0.7%, in order to furtherimprove the tempering resistance. Normally, however, the steel does notcontain any cobalt exceeding impurity level. Another element whichnormally need not exist in the steel, but which optionally may bepresent, is nickel, in order to improve the ductility of the steel. Attoo high contents of nickel, however, there is a risk of formation ofretained austenite. Therefore the nickel content must not exceed max.2.0%, preferably max. 1.0%, suitably max. 0.7%. If an effective contentof nickel is considered to be desired in the steel, the content e.g. mayamount to 0.30-0.70%, suitably to about 0.5%. In a preferred embodiment,when it is considered that the steel has a sufficientductility/toughness also without nickel, the steel, in relation to costreasons, should not contain nickel in amounts exceeding that content ofnickel which the steel unavoidably will contain in the form of animpurity from used raw materials, i.e. less than 0.30%. Further, thesteel in a manner per se, can optionally be alloyed with very smallcontents of different elements in order to improve the features of thesteel in various respects, e.g. its hardenability, or for facilitatingthe manufacturing of the steel. For example, the steel may optionally bealloyed with boron in contents up to about 30 ppm in order to improvethe hot ductility of the steel.

Other elements, on the other hand, are explicitly undesired. Thus, thesteel does not contain any other strong carbide formers than vanadium.Niobium, titanium, and zirconium, for example, are explicitly undesired.Their carbides are more stabile than vanadium carbide and require highertemperature than vanadium carbide in order to be dissolved at thehardening operation. While vanadium carbides begin to be dissolved at1000° C. and are in effect completely dissolved at 1100° C., niobiumcarbides do not start to be dissolved until at about 1050° C. Titaniumcarbides and zirconium carbides are even more stabile and do not startto be dissolved until temperatures above 1200° C. are reached and arenot completely dissolved until in the molten condition of the steel.Strong carbide and nitride formers other than vanadium, particularlytitanium, zirconium, and niobium, therefore must not exist in amountsabove 0.1%, preferably max. 0.03%, suitably max. 0.010%. Mostconveniently, the steel does not contain more than max. 0.005% of eachof said elements. Also the contents of phosphorus, sulphur, nitrogen andoxygen are kept at a very low level in the steel in order to maximisethe ductility and toughness of the steel. Thus, phosphorus may exist asan unavoidable impurity in a maximum amount of 0.035%, preferably max.0.015%, suitably max. 0.010%. Oxygen may exist in a maximal amount of0.0020% (20 ppm), preferably max. 0.0015% (15 ppm), suitably max.0.0010% (10 ppm). Nitrogen may exist in an amount of max. 0.030%,preferably max. 0.015%, suitably max. 0.010%.

If the steel is not sulphurised in order to improve the machinability ofthe steel, the steel contains max. 0.03% sulphur, preferably max. 0.010%S, suitably max. 0.003% (30 ppm) sulphur. However, one may conceive toimprove the machinability of the steel by intentional addition ofsulphur in an amount above 0.03%, preferably above 0.10% up to max.0.30% sulphur. If the steel is sulphurised, it may in a manner known perse also contain 5-75 ppm Ca and 50-100 ppm oxygen, preferably 5-50 ppmCa and 60-90 ppm oxygen.

During the manufacturing of the steel, there are produced ingots orblanks having a mass exceeding 100 kg, preferably up to 10 tons andthicknesses exceeding about 200 mm, preferably up to at least 350 mm.Preferably, conventional melt metallurgical manufacturing is employedvia ingot casting, suitably bottom casting. Also continuous casting maybe employed, provided it is followed by recasting to desired dimensionsaccording to above, e.g. by ESR remelting. Powder metallurgymanufacturing or spray forming are unnecessarily expensive processes anddo not give any advantages which motivate the cost. The produced ingotsare hot worked to desired dimensions, when also the cast structure isbroken down.

The structure of the hot worked material can be normalised in differentways by heat treatment in order to optimise the homogeneity of thematerial, e.g. by homogenisation treatment at high temperature, suitablyat 1200-1300° C. The steel is normally delivered by the steelmanufacturer to the customer in the soft annealed condition of thesteel; hardness about 160-220 HB, normally about 190 HB. The tools arenormally manufactured by machining operations in the soft annealedcondition of the steel, but it is also conceivable per se to manufacturethe tools by conventional machining operations or by spark machining inthe hardened and tempered condition of the steel.

The heat treatment of the manufactured tools is normally carried out bythe customer, preferably in a vacuum furnace, by hardening from atemperature between 950-1075° C., suitably at 1000-1050° C., forcomplete dissolution of existing carbides, for a period of time between15 min to 2 h, preferably for 15-60 min, followed by cooling to 20-70°C., and high temperature tempering at 500-570° C., suitably at 520-560°C. In the soft annealed condition of the steel, the steel has a ferriticmatrix containing evenly distributed, small carbides, which may be ofdifferent kind. In the hardened and not tempered condition, the steelhas a matrix consisting of untempered martensite. In terms ofcalculation by known theoretical calculations, the steel at equilibriumcontains about 0.6 vol-% MC carbides. At high temperature tempering, anadditional precipitation of MC carbides is obtained, which affords thesteel its intended hardness. These carbides have a sub microscopic size.The amount of carbides is therefore impossible to state by conventionalmicroscopic studies. If the temperature is increased too much, the MCcarbides are caused to be more coarse and become instable, which insteadcauses rapidly growing chromium carbides to be established, which is notdesired. For these reasons, it is important that the tempering isperformed at the above mentioned temperatures and holding times as faras the alloy composition of the steel of the invention is concerned.

Further features and aspects of the invention will be apparent from thepatent claims and from the following description of performedexperiments and from the subsequent discussion.

BRIEF DESCRIPTION OF DRAWINGS

In the following description of performed experiments, reference will bemade to the accompanying drawings, in which

FIG. 1 is a chart illustrating the hardness after hardening of examinedsteels versus the austenitising temperature,

FIG. 2 is a chart showing the hardness versus the tempering temperaturewithin a limited temperature range,

FIG. 3 is a chart illustrating the hardenability of examined steels,

FIG. 4 shows a diagram showing the ductility in terms of impact energyversus cooling time for samples hardened in vacuum furnace followed bytempering to about 55 HRC, and,

FIG. 5 and FIG. 6 are micro-photographs which at a large magnificationshow fracture surfaces of two examined steels.

DESCRIPTION OF PERFORMED EXPERIMENTS

Materials

Eight steel alloys were manufactured in the form of laboratory ingotshaving a mass of 50 kg. The chemical compositions of these ingots, whichwere manufactured at a laboratory scale, are given in Table 1, thesteels 1A-8A. The steels 1A-6A are experimental steels, while the steels7A and 8A are reference materials. In table 1 there are also given theaimed compositions, 1R-6R, of the experimental steels and the nominalcompositions, the steels 7N and 8N, of the reference materials, and alsoone of the commercial steels mentioned in the preamble, steel 9N. Thesulphur content of the 50 kg ingots could not be kept at a desirably lowlevel in the majority of the laboratory heats because of the limitationsof the manufacturing technique. In all the experimental steels, thecontent of titanium was in the order of 30 ppm and the content ofniobium in the order of 10 ppm. The content of zirconium was less than10 ppm. The following processing was applied: homogenisation treatment10 h at 127° C./air, forging to Ø60×60 nm, regeneration treatment at1050° C./2 h/air, and soft annealing at 850° C./2 h, cooling 10° C./h to600° C., then free cooling in air. TABLE 1 Chemical composition,weight-%, of experimental alloys and reference materials, balance Fe andunavoidable impurities Steel C % Si % Mn % P % S % Cr % Mo % V % N % O(ppm) 1R 0.42 0.20 0.50 <0.01 ≦0.005 5.00 2.30 0.35 — — 1A 0.41 0.220.47 0.004 0.006 4.97 2.33 0.36 0.016 71 2R 0.44 1.00 0.50 <0.01 ≦0.0055.00 2.30 0.35 — — 2A 0.43 0.88 0.46 0.004 0.006 4.97 2.29 0.37 0.013 713R 0.43 0.20 0.50 <0.01 ≦0.005 5.00 2.30 0.55 — — 3A 0.41 0.19 0.400.003 0.006 4.89 2.34 0.51 0.020 75 4R 0.44 0.20 0.50 <0.01 ≦0.005 5.002.30 0.52 — — 4A 0.43 0.11 0.44 0.004 0.004 4.80 2.32 0.48 0.02 93 5R0.48 0.20 0.50 <0.01 ≦0.005 5.00 2.30 0.52 — — 5A 0.46 0.11 0.45 0.0040.005 4.90 2.31 0.49 0.02 — 6R 0.48 1.00 0.50 <0.01 ≦0.005 5.00 2.300.55 — — 6A 0.47 0.98 0.47 0.004 0.006 5.13 2.32 0.55 0.017 64 7N 0.600.35 0.80 <0.02 ≦0.005 4.50 0.50 0.20 7A 0.59 0.32 0.72 0.004 0.006 4.440.54 0.28 0.013 59 8N 0.55 1.00 0.75 <0.02 ≦0.005 2.60 2.25 0.88 8A 0.521.01 0.71 0.004 0.006 2.68 2.25 0.87 0.016 60 9N 0.53 0.30 0.70 <0.02≦0.005 3.25 1.50 0.35R: Aimed composition of experimental alloysN: Nominal composition of reference materialsA: Analysed compositions of produced 50 kg heats

The above materials were examined with reference to hardness after softannealing, micro-structure after different heat treatments, hardnessafter hardening from different austenitising temperatures, hardnessafter tempering at different tempering temperatures, hardenability,impact toughness, and wear resistance. These investigations are reportedin the following. Moreover, theoretical equilibrium calculations werecarried out by the Thermo-Calc method with reference to the content ofdissolved carbon and carbide fraction at the indicated austenitisingtemperature for the steels which have the aimed compositions 1R-6R andthe nominal compositions 7N-9N of the reference steels, respectively,Table 2. TABLE 2 Contents of dissolved carbon in weight- %, at theaustenitising temperature, T_(A), and volume- % MC at T_(A) OptimalT_(A) Steel (° C.) % C at T_(A) % MC at T_(A) % M7C3 at T_(A) 1R 10200.41 0.14 — 2R 1020 0.41 0.42 — 3R 1020 0.38 0.56 — 4R 1020 0.39 0.52 —5R 1020 0.42 0.59 — 6R 1020 0.40 0.93 — 7N 960 0.52 0.13 1.23 8N 10500.39 1.67 — 9N 960 0.47 0.64 —Soft Annealed Hardness

The soft annealed hardness, Brinell hardness (HB), of the alloys 1A-8Ais given in Table 3. Table 1 and 3 show that a low silicon contentreduces the soft annealed hardness. TABLE 3 Soft annealed hardness SteelHardness (HB) 1A 174 2A 199 3A 176 4A 171 5A 181 6A 212 7A 191 8A 222Micro-Structure

The micro-structure was examined in the soft annealed condition andafter heat treatment to hardnesses between 55 and 58 HRC of the alloys1R-8R. The micro-structure consisted of tempered martensite in thehardened and tempered condition of the steels. No primary carbides werepresent. Nor could any titanium carbides, nitrides and/or carbonitridesbe detected in any alloy.

Hardening and Tempering

The steels 1A-6A were austenitised by heating for 30 minutes atdifferent temperatures between 1000 and 1050° C., while the referencesteels 7A and 8A were austenitised for 30 minutes at 960° C. and 1050°C., respectively, which are the optimal austenitising temperatures ofthese known steels. The influence of the austenitising temperature uponthe hardness of the steels 1A-6A is shown in FIG. 1, where also thehardness of the reference materials 7A and 8A after said austenitisingtreatment is shown.

The influence of the tempering temperature on the hardness of the steels1A-8A after austenitising at 1025° C. of the steels 1A-6A, at 960° C. ofthe steel 7A, and at 1050° C. of the steel 8A, 30 min, was examined. Atypical secondary hardening was observed at a temperature between 450°C. and 600° C. for all the steels except for steel 7A. FIG. 2 shows thehardness versus the tempering temperature within the interestingtemperature range between 500° C. and 600° C. All the steels weretempered 2×2 h at the indicated temperatures. Steel 6A exhibited thebest tempering resistance of the examined materials up to a temperingtemperature of 550° C. Steel 2A had a tempering resistance which wasequally as good as that of the reference material 8A up to 525° C.,while the steels 1A and 3A-5A had a wear resistance on a level lowerthan the tempering resistance of steel 8A but significantly higher thanthe tempering resistance of steel 7A. The tempering resistance of theexperimental alloys 1A-6A therefore may be considered to be good, whichis important for a matrix steel which may require surface coating at atemperature up to about 500° C. in order to obtain a wear resistancenecessary for some tool applications. In other words, at a temperaturebetween 450° C. and 600° C., more exactly at a temperature between 500°C. and 560° C., a pronounced secondary hardening is obtained byprecipitation of MC carbides. The wear resistance is favoured by a highsilicon content, but also if the silicon content is low, such as insteel 5A, a hardness above 56 HRC can be maintained after hightemperature tempering up to about 540° C. This is advantageous, becauseit makes it possible to perform the surface treatment within a ratherwide temperature range without causing the hardness of the tool to betoo low.

Hardenability

A comparison of the hardenability in terms of Vicker hardness (HV10)versus the time required for cooling from 800-500° C., using plotteddata from CCT diagrams, is shown in FIG. 3 for the examined alloys1A-8A. As is apparent from the chart, all experimental alloys 1A-6A havea better hardenability than the reference steels 7A and 8A. Especiallysteel 5A has a very good hardenability, while the reference material 8Aachieves only 52 HRC in the hardened condition at t₈₋₅=1000 s. Thereference steel 7A reaches 55 HRC, while all experimental alloys 1A-6Areach a hardness>56 HRC at said cooling rate.

Ductility

The ductility in terms of absorbed impact energy for un-notched testrods at 20° C. is shown in FIG. 4 for rods of the alloys 1A-8A cooled ina vacuum furnace versus the cooling time from 800° C. to 500° C. Theshown cooling times are realistic cooling times for full size mouldtools for plastic moulding. All steels are tempered to an aimed value of55 HRC. The best ductility was obtained by the experimental alloys 3A,4A, and 5A, which contain about 0.1% to about 0.2% Si and about 0.5% V.This is also shown in Table 4, which shows the ductility in terms ofabsorbed impact energy for un-notched test rods at 20° C. hardened in avacuum furnace and cooled at a rate corresponding to t₈₋₅=1190 s andtempered to a hardness of 55±0.8 HRC. Corresponding variants having alower content of vanadium have a lower ductility. Comparative studies offracture surfaces show that the variants with the lower vanadium contenthave larger austenite grain sizes, FIG. 5, which can be explained by thefact that these alloys contain a lower content of austenite grain growthpreventing vanadium carbides in the matrix than those variants whichhave a slightly higher content of vanadium. FIGS. 5 and 6 show fracturesurfaces of test rods made of the alloys 1A and 3A, respectively. Themicrophotograph in FIG. 6 shows a ductile fracture of a test rod made ofa steel with an adequate alloy composition according to the invention,which has a fine austenite grain size, which is a prerequisite for agood ductility. TABLE 4 Ductility in terms of absorbed impact energy inthe transversal direction for un-notched test rods at 20° C.; hardness55 ± 0.8 HRC Steel Ductility (J) 1A 195 2A 80 3A 245 4A 255 5A 275 6A180 7A 175Wear Resistance

A pin against pin test with SiO₂ as an abrasive wear agent was carriedout for the examined alloys 1A-8A. Steel 7A had the lowest wearresistance. At comparable hardnesses, the other steels had an equallygood wear resistance. Those alloys which had a higher silicon content,however, had a somewhat better wear resistance.

Discussion

The intention of the work carried out in connection with the developmentof the present invention is to achieve a steel having a desiredcombination of features as indicated in the left column in Table 5. Inthe table the marks 1-3 are used, where 1=lowest and 3=best. Theexperimental alloy which comes nearest the ideal is steel 5A. This steelhas been compared with the reference material 8A. No serious drawbacks,but many advantages, in the view of its use for mould tools for plasticmoulding could be registered for the steel 5A in this comparison. Incomparison with the reference material 7A, it is an important advantagethat the steel can be high temperature tempered, while steel 7A requireslow temperature tempering with the known drawbacks which this gives inconnection with spark machining, retained high tensions after heattreatment, and restrictions as far as the choice of surface treatment isconcerned. The marks for fatigue life are calculated with reference tothe cleanliness of the steels. The pressure strength is calculated onthe basis of the tempering temperature and the hardness of the materialsafter tempering. Grindability, machinability, and polishability havebeen calculated on the basis of the ductility, the soft annealedhardness, and the carbide content of the materials. The weldability isrelated to the carbon content and to the content of alloy elements. Theproduction economy has been considered with reference to the possibilityto manufacture the steels in a conventional way without problems. TABLE5 Desired combination of features; comparison of features of examinedsteels Desired Parameters/ combination Steel Steel Steel Features offeatures 8A 7A 5A Hardenability 3 1 2 3 Dimension stability at 3 1 2 3heat treatment Hardness after 3 3 3 3 tempering (56-58 HRC) Impacttoughness 3 2 1 3 Wear resistance 2 2 3 3 Fatigue life 3 3 3 3 Pressurestrength 3 3 3 3 Grindability 3 3 3 3 Machinability 3 3 3 2 Sparkmachinability 3 3 2 3 Weldability 2 2 1 2 Polishability 3 3 3 3Production economy 3 3 2 3

In comparison with the ideal combination of features, steel 5A has asomewhat low hardness after hardening and high temperature tempering. Onthe basis of the experiences obtained by the experiments, it isestimated that the silicon content of an optimal steel compositionshould be about 0.2% and that the content of dissolved carbon at 1020°C. in such a steel should be about 0.45%. The silicon content, however,should not exceed 0.25% in the optimal composition in order to providean optimal ductility/toughness of the alloy. The aimed value of thecarbon content of the steel, in that case should be 0.49% in order togive an aimed hardness of 57-58 HRC after hardening and high temperaturetempering. A suitable vanadium content of the optimal composition isestimated to be 0.52% in order to give a wider margin against graingrowth in connection with the heat treatment. The contents ofphosphorus, sulphur, nitrogen, and oxygen are kept at a very low levelin order to maximise ductility and toughness. The steel shall notcontain any other, intentionally added carbide formers than vanadium.Other carbide formers, such as titanium, zirconium, and niobium are eachlimited to max. 0.005% in the optimal alloy. Aluminium may be present asa residual from the manufacturing of the steel and is limited to max.0.030, preferably to max. 0.015%.

An optimal alloy for mould steels for plastic moulding therefore shouldhave the composition which is given in Table 6.

Production Scale Experiments

A steel 10P according to the invention was manufactured in an electricarc furnace. The aimed composition was the composition according toTable 6. The heat had a weight of 65 tons. The analysed composition onlydiverged very little from the aimed composition. The only elements whichwere outside of the given norm were sulphur and nitrogen, the contentsof which amounted to 0.011% and 0.013%, respectively, instead of max.0.010%. The complete composition of the steel 10P is given in Table 7,in which also the content of the most important impurities are stated.In the same table, also the composition of three examined referencematerials, 7P, 8P, and 9P, taken from the applicant's production, arestated. These steels correspond to the steels 7N, 8N, and 9N, which havethe nominal compositions stated in Table 1. Also the reference materialswere manufactured as 65 ton heats in an electric arc furnace. All theheats were bottom casted to the shape of ingots. The ingots which weremanufactured of steel 9P were also refined by ESR remelting. The ingots,including the ESR ingots, were forged to the shape of bars havingdifferent dimensions. The bars were subjected to different heattreatments before test samples were taken out. The dimensions and heattreatments of the examined bars are given in Table 8.

Then three more production heats with chemical compositions according tothe invention, each of 65 tons, were manufactured in the electric arcfurnace. From the steels, there were produced electrodes, which weresubjected to ESR (Electro Slag Refining). The ESR ingots were forged tothe shape of bars with different dimensions. These bars were alsosubjected to different heat treatments before test samples were takenout. Also the chemical compositions of these bars, the steels 11P, 12P,and 13P, are given in Table 7 and their dimensions and heat treatmentsin Table 8. TABLE 8 Bar dimensions and heat treatments Steel No Bardimension, mm Heat treatment 7P Ø315 T_(A) 960° C., 30 min Tempering200° C., 2 × 2 h 8P Broad flat bar, T_(A) 950° C., 30 min Thickness 102mm Tempering 200° C., 2 × 2 h 9P Ø330 mm T_(A) 1050° C., 30 minTempering 575° C., 2 × 2 h 9P Flat bar, T_(A) 1050° C., 30 min 350 × 127mm Tempering 575° C., 2 × 2 h 10P Ø350 mm T_(A) 1025° C., 30 minTempering 525° C., 2 × 2 h 10P Flat bar, T_(A) 1025° C., 30 min 396 ×136 mm Tempering 525° C., 2 × 2 h 11P Flat bar T_(A) 1020° C., 30 min396 × 136 mm Tempering 525° C., 2 × 2 h 12P Ø350 mm T_(A) 1000° C., 30min Tempering 550° C., 2 × 2 h 13P Flat bar T_(A) 1000° C., 30 min 596 ×346 mm Tempering 550° C., 2 × 2 h

TABLE 6 Optimal alloy composition, weight-%, content of dissolved carbonand carbide content at 1020° C. MC C Si Mn P S Cr Mo V Al N O C Vol-%Min. 0.46 0.10 0.40 — — 4.85 2.20 0.47 — — — 0.42 0.51 Aimed 0.49 0.200.50 ≦0.010 ≦0.0010 5.00 2.30 0.52 ≦0.015 ≦0.010 ≦0.0008 0.44 0.56 valueMax. 0.51 0.25 0.60 ≦0.010 ≦0.010 5.15 2.40 0.57 ≦0.030 ≦0.010 ≦0.00080.46 0.59

TABLE 7 Chemical composition, weight-%, and weight-ppm, respectively, ofexamined production scale steels, balance Fe and impurities Steel P S NiW Co Ti Nb Al N B O No C % Si % Mn % ppm ppm Cr % % Mo % ppm ppm V % ppmppm ppm ppm ppm ppm 7P 0.59 0.34 0.81 80 33 4.59 0.07 0.49 100 n.a. 0.2510 20 250 170 n.a. <12 8P 0.53 0.34 0.68 190 20 3.11 0.09 1.53 n.a. n.a.0.04 20 <20 160 80 n.a. 9 9P 0.55 1.02 0.74 140 2 2.60 0.08 2.23 n.a.n.a. 0.83 <20 <20 410 80 23 <12 10P 0.51 0.22 0.44 70 11 5.03 0.08 2.3220 10 0.50 25 <10 260 130 1 8 11P 0.48 0.19 0.48 70 6 5.00 n.a. 2.31n.a. n.a. 0.50 n.a. n.a. 160 160 n.a. 10 12P 0.46 0.18 0.48 70 5 4.960.06 2.27 30 90 0.50 17 10 60 100 1 14 13P 0.51 0.13 0.48 80 3 5.02 0.062.34 20 80 0.51 16 10 90 110 1 8n.a. = not analysed

The samples which were taken out from the bars according to Table 8 wereexamined with reference to hardness and impact toughness. The resultsare stated in Table 9. In this table also the kind of test rod (all thetest rods were un-notched) and the position of the test rod in the barare stated.

-   CL2 means test rod from a round bar, taken in the centre of the bar    in the longitudal direction of the bar and with the impact direction    in the square direction of the bar,-   CR2 means the same as CL2 but with the impact direction in the    longitudal direction of the bar (most unfavourable conditions),-   TL2 means test rod from a flat bar and in other respects according    to CR2,-   LT2 means test rod from a flat bar and in other respects according    to CL2, and

ST2 means test rod from a flat bar, taken out from the centre of thebar, in the shortest square direction and with the impact direction inthe longitudal direction (most unfavourable conditions). TABLE 9Hardness and impact toughness of examined steels manufactured at aproduction scale Steel No, type of test rod Impact and positionHardness, HRC toughness, J 7P, CL2 58 42 8P, TL2 57 83 9P, CL2 58 60 9P,TL2 58 159 10P, CR2 57.5 58 10P, TL2 57.5 196 11P, LT2 55.9 336 11P, ST255.9 216 12P, CR2 57 285 13P, ST2 57.7 239

As is shown in Table 9, the hardnesses of the examined steels wereequally good, but required, as far as steels 7P and 8P are concerned,low temperature tempering with its known drawbacks. The comparativelygood impact toughness of steel 8P, however, in the first place must beattributed to the thinner dimension of the examined flat bar made ofthat steel. For steel 9P, only a moderately good impact toughness wasachieved, although the steel was ESR refined. The measured value of theimpact toughness of the round bar of steel 10P, 58 J, was only slightlylower than the measured value of the impact toughness of the round barof steel 9P, 60 J, in spite of the unfavourable impact direction. It canfurther be observed, that in the case of equal tests of the impacttoughness of the flat bars of the steels 9P and 10P, the clearly bestimpact toughness, 196 J, could be noted for the steel 10P according tothe invention, which shall be compared with 159 J for steel 9P. In thiscomparison, it should particularly be considered that the 9P steel wasESR refined, which normally improves the toughness. Finally it may benoted that the impact toughness of the steels 11P, 12P, and 13P of theinvention, have been strongly improved by the ESR remelting as comparedwith the non ESR remelted material, steel 10P.

1-32. (canceled)
 33. Steel, characterised in that it has the followingchemical composition in weight-%: 0.43-0.60 C from traces to 1.5 Si fromtraces to 1.5 (Si+Al) 0.1-2.0 Mn 3.0-7.0 Cr${1.5 - {4.0\quad\left( {{Mo} + \frac{W}{2}} \right)}},$ however max 1.0W 0.30-0.70 V max. 0.1 of each of Nb, Ti and Zr max. 2.0 Co max. 2.0 Nibalance essentially only iron and unavoidable impurities.
 34. Steel,characterised in that it has the following chemical composition inweight-%: 0.43-0.60 C from traces to 1.5 Si from traces to 1.5 (Si+Al)0.1-2.0 Mn 3.0-7.0 Cr${1.5 - {4.0\quad\left( {{Mo} + \frac{W}{2}} \right)}},$ however atleast 2.1 Mo and max 1.0 W 0.30-0.70 V max. 0.1 of each of Nb, Ti and Zrmax. 2.0 Co max. 2.0 Ni balance essentially only iron and unavoidableimpurities.
 35. Steel according to claim 33, characterised in that itcontains at least 0.44, suitably at least 0.46 C.
 36. Steel according toclaim 35, characterised in that it contains max 0.55, suitably max. 0.53C.
 37. Steel according to claim 33, characterised in that it contains atleast 0.40, suitably at least 0.45 V.
 38. Steel according to claim 37,characterised in that it contains max 0.65, suitably max. 0.60 V. 39.Steel according to claim 33, characterised in that it contains about0.49 C and about 0.52 V.
 40. Steel according to claim 33, characterisedin that it contains at least 0.05 and max. 1.0 Si.
 41. Steel accordingto claim 40, characterised in that it contains at least 0.1 and max. 0.5Si.
 42. Steel according to claim 41, characterised in that it containsnominally 0.2 Si.
 43. Steel according to claim 33, characterised in thatit contains max. 1.0, preferably max 0.3, suitably max. 0.1 and mostconveniently max. 0.03 Al.
 44. Steel according to claim 33,characterised in that it contains max. 3.2 Mo.
 45. Steel according toclaim 44, characterised in that it contains max 2.6 Mo.
 46. Steelaccording to claim 44, characterised in that it contains max. 0.3,suitably max. 0.1 W.
 47. Steel according to claim 46, characterised inthat it does not contain tungsten exceeding impurity level.
 48. Steelaccording to claim 33, characterised in that it contains max. 0.7 Co.49. Steel according to claim 48, characterised in that it does notcontain cobolt exceeding impurity level.
 50. Steel according to claim33, characterised in that it contains max. 1.0 Ni.
 51. Steel accordingto claim 50, characterised in that it contains max. 0.7 Ni.
 52. Steelaccording to claim 51, characterised in that it contains 0.3-0.7 Ni. 53.Steel according to claim 52, characterised in that it does not containnickel exceeding impurity level.
 54. Steel according to claim 33,characterised in that the content of each of the elements titanium,zirconium and niobium does not exceed 0.01%.
 55. Steel according toclaim 54, characterised in that the content of each of the elementstitanium, zirconium and niobium does not exceed 0.03%.
 56. Steelaccording to claim 55, characterised in that the content of each of theelements titanium, zirconium and niobium does not exceed 0.01%,preferably does not exceed 0.005%.
 57. Steel according to claim 33,characterised in that the steel does not contain more than max. 0.035%,preferably max. 0.015% and suitably max. 0.010% P.
 58. Steel accordingto claim 33, characterised in that the steel contains max. 20 ppm,preferably max. 10 ppm O.
 59. Steel according to claim 33, characterisedin that the steel contains max. 30 ppm, preferably max. 15 ppm, andsuitably max. 10 ppm N.
 60. Steel according to claim 33, characterisedin that it contains max. 0.03%, preferably max. 0.01%, and suitably max.30 ppm S.
 61. Steel according to claim 33, characterised in that itcontains 0.10-0.30% S.
 62. Steel according to claim 61, characterised inthat it contains 5-75 ppm Ca and 50-100 ppm O, preferably 5-50 ppm Caand preferably 60-90 ppm O.
 63. Steel according to claim 33,charactrised in that it after hardening and high temperature temperingat 500-570° C., preferably at 520-560° C., has a hardness of 54-59 HRC,preferably 56-68 HRC.
 64. Steel according to claim 33, characterised inthat it is ESR remelted.
 65. Mould tool for plastic moulding,manufactured of steel according to claim
 33. 66. Mould tool for plasticmoulding according to claim 65, characterised in that it after hardeningand high temperature tempering at 500-570° C., preferably at 520-560°C., has a hardness of 54-59 HRC, preferably 56-58 HRC.